1. Field of the Invention
The present invention relates to a recording medium, such as an optical disk or optical card and an apparatus and method for manufacturing the recording medium.
2. Description of the Related Art
Recordable recording media, particularly a write once DVD-R (Digital Versatile Disc-Recordable) and rewritable DVD-RW (Digital Versatile Disc-Re-recordable) (hereinafter those media will be generically called DVDs) have already been manufactured. Previously recorded on a DVD are address information needed to look for a target position at the time of recording data, such as image information, and rotation control information, such as a wobbling signal, which is used in controlling the rotation of a disk. (Hereinafter, those information will be generically called pre information.)
The rotation control information is recorded by previously wobbling data-recording tracks (groove tracks or land tracks) to a waveform of a given amplitude at a predetermined frequency (wobbling frequency) in the preformat phase at the time of manufacture.
At the time of actually recording data on a DVD, therefore, the wobbling frequency of the wobbled tracks is detected, a reference clock for controlling the rotation of the DVD is extracted based on the wobbling frequency, and a drive signal for controlling the rotation of the spindle motor that rotates the DVD based on the extracted reference clock and a recording clock signal including timing information which is synchronous with the rotation of the DVD are generated.
Further, address information indicating an address on the DVD that is needed at the time of recording data is recorded by forming prepits corresponding to the pre information is formed on a track lying between two tracks (e.g.,a land track). Further, the prepits are formed nearly uniformly over the entire surface of the DVD so that the reference clock can be reproduced even from the prepits as needed.
FIG. 1 shows one example of a recording layer on a DVD and the cross section of the DVD. As illustrated, convex groove tracks GV and recessed land tracks LD are previously and alternately formed on the recording layer of, for example, a phase change material on the DVD spirally or concentrically. That is, pairs of both tracks are repeatedly laid side by side.
Formed on the land tracks LD beforehand are addresses indicating positions on the groove tracks GV and relating information, such as a plurality of land prepits LPP which carry the recording timing. Each of the land prepits LPP is formed in such a way as to couple both adjoining groove tracks GV, and the surface of each land prepit LPP is level with the surfaces of the associated groove tracks GV.
FIG. 1 shows a mode before data to be recorded (audio data, video data and computer data) by an information recording and reproducing apparatus is recorded. While FIG. 1 linearly shows the individual groove tracks GV, the groove tracks GV are actually wobbled at a frequency corresponding to the rotational speed of the DVD. That is, pairs of the land tracks LD and groove tracks GV are provided side by side while being bent periodically.
The information recording and reproducing apparatus that records data on a DVD irradiates and focuses a recording light beam according to data on a groove track GV as shown in FIG. 2 while identifying a position on the groove track GV by detecting a land prepit LPP from the DVD. At this time, the portion where the recording light beam has been irradiated is heated, thereby forming a record marker portion M whose reflectance differs from the ambient reflectance on that portion of the groove track GV. As a land prepit LPP that carries information, such as an address, about one groove track is formed on the outer track side of the groove track, a land prepit LPP on the outer track side of each track is detected as shown in FIG. 2.
The information recording and reproducing apparatus has a prepit detecting unit which detects a land prepit LPP. The prepit detecting unit includes a 4-split photodetector 1 as shown in FIG. 3. The 4-split photodetector 1 is comprised of a photoelectric converting device having four light-receiving surfaces 1a to 1d separated into four segments by a direction along the groove tracks GV of the DVD and a direction perpendicular to the groove tracks. The light-receiving surfaces 1a and 1d are positioned on the outer track side of the DVD while the light-receiving surfaces 1b and 1c are positioned on the inner track side of the DVD.
A read light beam generator irradiates a read light beam on the DVD that is rotated by the spindle motor, thereby forming a beam spot on the recording layer. The photoelectric converting device detects reflected light of the information reading spot from the DVD at the four light-receiving surfaces 1a–1d and outputs reception signals Ra to Rd or electric signals corresponding to the amounts of light respectively detected by the light-receiving surfaces 1a–1d. The reception signals Ra and Rd associated with the light-receiving surfaces 1a and 1d positioned on the outer track side of the DVD are supplied to an adder 2, and the reception signals Rb and Rc associated with the light-receiving surfaces 1b and 1c positioned on the inner track side of the DVD are supplied to an adder 3. The adder 2 adds the light-receiving surfaces Ra and Rd, and the adder 3 adds the light-receiving surfaces Rb and Rc. Further, a subtracter 4 subtracts the output signal of the adder 3 from the output signal of the adder 2, and provides an output signal as a radial push-pull signal.
When the irradiated beam spot is on a position centering a groove track GV carrying no data in center and including a land prepit LPP as shown in FIG. 2, the diffraction of the light beam reduces the amounts of reflected light to the light-receiving surfaces 1a and 1d of the photodetector 1 and increases the amounts of reflected light to the light-receiving surfaces 1b and 1c. As a result, the level of the output signal of the adder 2 falls below the level of the output signal of the adder 3. Therefore, the radial push-pull signal output from the subtracter 4 in association with the position of the land prepit LPP has a waveform with a sharp trough as shown in FIG. 4. The radial push-pull signal is supplied to a binarization circuit 5 and is binarized with a predetermined threshold value to detect the land prepit LPP.
When the recording light beam is irradiated on a land prepit LPP to form the data-carrying record marker portion M, the heat generated by the irradiation of the recording light beam is transferred to a part of the land prepit LPP from the groove track GV. This forms a record marker portion M1 with a larger area than that of the record marker portion M of the groove track in a land-prepit absent region as shown in FIG. 2.
When information data is reproduced from the data-recorded DVD, therefore, the waveform of a read signal obtained at the time of reading the record marker portion M1 near the land prepit LPP may have distortion, which leads to a higher read error ratio.